Member Of 2D Materials Research Articles

Emerging 2D Cobalt Telluride (CoxTey): from Theory to Applications

AbstractCobalt telluride, with tunable magnetism and ferromagnetism that hinged upon the stoichiometric ratio, has emerged as a new member of 2D materials in the last decade. Metallic doping by sodium (Na) and platinum (Pt) atoms below critical concentrations is found to enhance the magnetism of cobalt telluride. After thinning cobalt telluride down to few‐layer, the saturation magnetism is improved by two order of magnitudes because of the oxidation state of cobalt (Co) and reduced coordination number of the surface atoms. In 2D limit, cobalt di‐telluride possesses Dirac band structure with many nodal lines that correlate with quantum criticality and other fantastic physics. In this mini‐review, the crystal and band structures of cobalt telluride categorized by stoichiometric ratio are overviewed after briefing the introduction. Both top‐down and bottom‐up methods are then discussed to offer optimum solutions for each specified application scenario. Afterward, emerging magnetic and electronic properties and credit enhancements are launched accompanied with their key advances. Beyond these, the faced challenges and possible directions of future research are also provided, attempting to boost both fundamental physics and device applications based on 2D cobalt telluride.

Revisiting four-phonon scattering in WS2 monolayer with machine learning potential

Recently, the importance of the higher-order anharmonic effect on the thermal transport has been demonstrated in a few two-dimensional (2D) materials. As a member of 2D materials, the WS2 monolayer possesses excellent properties, but its higher-order phonon anharmonic effect is still unclear. As an emerging tool for atomistic simulations, the machine learning Gaussian approximation potential (GAP) has good computational speed and accuracy. In this work, the effect of the four-phonon scattering on the thermal transport properties of the WS2 monolayer is systematically investigated by combining a well-trained GAP with the Boltzmann transport equation. We find that the in-plane thermal conductivity of the WS2 monolayer decreases by 34.68% at 300 K compared to the results without considering the four-phonon scattering, which mainly originates from the strong low-frequency redistribution scattering process. The temperature-induced phonon renormalization in WS2 monolayer is investigated using the temperature-dependent effective potential method. The results show that phonon modes slightly soften with increasing temperature, and the temperature effect weakens the four-phonon scattering while hardly affect the three-phonon scattering. Our work reveals that previous studies considering only the three-phonon scattering and the temperature effect are insufficient, and the four-phonon interaction must be introduced to accurately describe the thermal transport properties of the WS2 monolayer.

Tunable soliton molecules in mode-locked fiber laser based on GeBi4Te7 saturable absorber

As a member of 2D materials, GeBi4Te7 (GBT) nanoflakes about 3 nm thickness were fabricated using liquid-phase exfoliation. The GBT-SA has excellent nonlinear optical effects that the modulated depth and saturation power are 6.6 % and 1.07 mW measured by the double-balance detection method. The single-, dual-, triple-, fifth-, sixth-, and tenth-pulses molecules have been recorded under different pump power in the cavity length of 26.5 m, 16.7 m and 13.5 m cavities. These multiple soliton molecules are state-tunable by only adjusting pump power. The experimental results demonstrate the GBT has excellent nonlinear optics effects and broad application prospects in fiber communication and ultrafast lasers.

Room‐Temperature Non‐Local Spin Transport in Few‐Layer Black Phosphorus Passivated with MgO

AbstractBlack phosphorus (BP), a new member of 2D materials, is an ideal selection to construct spin‐based devices due to its tunable direct bandgap and high carrier mobility. Assembling van der Waals heterostructures is the most popular method to create spintronic devices for 2D materials, especially for the easily oxidized BP. However, it is too complicated to be realized for fabricating large‐scale integrated circuits in practical applications. To overcome this flaw, an oxide layer on BP simultaneously serving as the protection layer and barrier to fabricate a Co/MgO/BP‐based non‐local spin valve is employed. The non‐local spin signals demonstrate the diffusion of pure spin current in the BP channel, which is the direct evidence of the spin injection from Co into BP. Combining the Hanle precession measurements with the Bloch equation fitting, the spin transport parameters of the few‐layer BP can be extracted. The spin diffusion length λs and spin relaxation time τs are 6.15 µm and 241.7 ps, respectively. Therefore, the MgO layer in the non‐local spin valve can simplify the fabrication of 2D material‐based spintronic devices and accelerate their applications.

Study of the Growth Mechanism of Solution-Synthesized Symmetric Tellurium Nanoflakes at Atomic Resolution.

As a new member of 2D materials, 2D tellurium (Te) has recently attracted much attention due to its intriguing properties. Through hydrothermal processing, 2D Te with tunable thickness and size has been realized, and its growth mechanism has also been studied. However, the tailored growth of 2D Te nanoflakes with symmetrical morphologies and interfacial moiré fringes has never been reported. Here, 2D Te nanoflakes have been prepared using the hydrothermal method, and mirror-symmetrical shapes (including "V-shape," "heart-shape," and "paper airplane-shape") with obvious moiré fringes in the middle of the nanoflakes are observed. Comprehensive transmission electron microscopy (TEM) techniques are utilized for structural characterization of these nanoflakes, especially the moiré fringes in the symmetry axis region of the nanoflakes. The systematic analyses of the moiré fringes and the observation of obvious overlapping edges of the composing nanoflakes from the cross-sectional samples reveal the possible mechanism of morphological evolution for these symmetrical nanoflakes. These details may fill the research gap in the controllable growth of 2D Te nanomaterials, pave the way for the fabrication of 2D Te moiré superlattices and in-plane homojunctions, and promote their future versatile applications.

In‐Plane Optical Anisotropy of Low‐Symmetry 2D GeSe

AbstractAs a new member of 2D materials, GeSe has attracted considerable attention recently due to its fascinating in‐plane anisotropic vibrational, electrical, and optical properties originating from the low‐symmetry crystal structure. Among these anisotropic properties, the anisotropic optical property, as a new degree of freedom to manipulate optoelectronic properties in 2D materials, is of great importance for practical applications. However, the fundamental understanding of the optical anisotropy of GeSe is still under exploration, severely restricting its utility in polarization‐sensitive optical systems. Here, a systematic study about the in‐plane optical anisotropy of GeSe is reported, including its anisotropic optical absorption, reflection, extinction, and refraction. The anisotropic band structure of GeSe is experimentally observed for the first time through angle‐resolved photoemission spectroscopy, explaining the origin of the optical anisotropy. The anisotropic reflection and refraction of GeSe are further directly visualized through the angle‐dependent optical contrast of GeSe flakes by azimuth‐dependent reflectance difference microscopy and polarization‐resolved optical microscopy, respectively. Finally, GeSe‐based photodetectors exhibit a polarization‐sensitive photoresponsivity due to the intrinsic linear dichroism. This study provides fundamental information for the optical anisotropy of GeSe, forcefully stimulating the exploration of novel GeSe‐based optical and optoelectronic applications.

Electrochemical prepared phosphorene as a cathode for supercapacitors

As a new member of 2D materials, the phosphorene possesses many unique characteristics compared with other carbon-based materials, which is strongly correlated to the structure and the electronic properties of phosphorus, can enable its use in electrochemical energy storage devices. Herein, we present a simpler approach to prepare phosphorene and then use it as an electrode in a new supercapacitor, which has a battery performance but works via the redox reaction of iodide ions on the interface of phosphorene electrode and electrolyte. The phosphorene is prepared by an electrochemical cathodic exfoliation method and the exfoliation mechanism is discussed. The high-quality phosphorene can be obtained within 5 min without ultrasonication and the yield is ∼93.1%. The dimension of phosphorene reaches micrometer grade. By virtue of the merit of the great electrocatalysis effect of phosphorene to iodide ions, the specific discharge capacity reaches to 3181.5 F/g in a three-electrode system and the largest specific energy density is 203.7 Wh/kg in an asymmetric hybrid system.

Ambipolar transport based on CVD-synthesized ReSe2

Control of the carrier type in two dimensional (2D) materials is a serious issue for the realization of logic devices. The carrier type control of 2D semiconducting materials such as MoTe2, WSe2 and black phosphorus have been studied for this purpose. However, the systematic study on the polarity control of transistors based on ReSe2, a new member of 2D materials, has remained unexplored despite the intriguing anisotropic optical and electrical properties deriving from the exotic crystal structure. Here, we report the electrical characterization of field effect transistors (FETs) of single crystalline ReSe2 grown by a chemical vapor deposition. In contrast to a previous report of unipolar p-type exfoliated crystals, synthesized ReSe2 FETs on SiO2 with Au contact exhibit highly symmetric ambipolar behaviors with the current on/off ratios of ~104 for both of hole and electron injection. The carrier type could be controlled via the metal contact. With Al contacts, ReSe2 FETs display perfect transition to pure n-type unipolar behavior. It is found that carrier type of ReSe2 via thickness variation was hardly modulated because the ReSe2 bandgap has little dependence on its thickness. We successfully achieved the fabrication of a logic inverter by using only ambipolar ReSe2 FETs on SiO2/Si without electrostatic doping or chemical treatments. These results demonstrate that ReSe2 is a promising candidate for future low power logic devices and functional nano electronic applications.